Render locale and plugin-bundled text as DokuWiki syntax

Static "intro" text shipped with DokuWiki, templates and plugins —
inc/lang/*/*.txt, template locale files, each plugin's lang/<lc>/*.txt —
i

Render locale and plugin-bundled text as DokuWiki syntax

Static "intro" text shipped with DokuWiki, templates and plugins —
inc/lang/*/*.txt, template locale files, each plugin's lang/<lc>/*.txt —
is authored in DokuWiki syntax. It is a core/plugin asset, not user
content. When an admin configures the wiki for Markdown the DW link and
monospace modes are not loaded, so these files render as literal text:
[[wiki:syntax]] and ''Save'' pairs survive into the HTML.

Pin those entry points to the 'dw' flavour via the override added in the
previous commit:

- p_locale_xhtml() and tpl_locale_xhtml() pass 'dw'.
- PluginTrait::locale_xhtml() passes 'dw'.
- PluginTrait::render_text() / PluginInterface::render_text() gain a
$syntax parameter defaulting to 'dw'. The default is 'dw', not null,
because the method predates GFM and its callers pass DW-syntax strings;
plugins rendering user content opt back into the configured syntax with
render_text($text, 'xhtml', null).

Locale output is now deterministic across a syntax switch, so its caches
get over-invalidated but never under-invalidated — acceptable, as the
locale render path is rare and cheap.

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